Present invention relates to an anaerobic digestive process which is suitable for a quick treatment of organic wastes, particularly those containing much solids.
Presently most of such organic wastes as excess activated sludge which is by-produced at the time of sewage treatment, agricultural and livestock waste and city garbage are being incinerated or landfilled or dumped. Among others, those wastes which contain much solids are not easy to dispose, and in many instances they are causing such problems a pollution by stack smoke, stench, water contamination, soil contamination and so forth.
Recently, general attention is being focused upon the anaerobic digestive process, as a non-polluting and energy-saving process which will replace the aforementioned procedures. This process has conventionally been employed for treating the collected human wastes in Japan and for treating the sludge used for treating sewage wastes in other countries. However, it has the disadvantage that it requires a rather extended period of time, 20 to 40 days, to complete the digestion. Therefore, if the process could be made speedier, it would be a very useful process. On the other hand, even for anaerobic digestion, human and animal wastes, agricultural and livestock wastes as well as city garbage are the materials which are difficult to dispose. As practical examples, (1) Only a very limited part of cellulose, pectin and hard protein, which are the main ingredients of the solids, is decomposed, and the most of them remain intact within the digestive sludge without being decomposed before discharge out of the system. Of course, the same applies to city garbage which involves such variety of admixture as waste plastics, sand and dirt, and numerous pieces of glass. (2) These solids burden the transmission system of the charged slurry and the stirring system installed in the digestion tank, which results in problems in the treatment system. In particular, it encourages a generation of scum in the digestion tank and cause a deterioration in the mixing state of the liquor, so that the number of days required for digestion will have to be extended which prevents a satisfactory methane fermentation. Further, it causes more power consumption for stirring and requires additional labor for taking out the scum for disposal.
In the past situations of disposing of collected human wastes and in experiments for treating city garbage disposal, solids were anaerobically digested after being crushed into pieces having a certain particle size, but in either cases the aforementioned points were the problem. Although there is an example wherein the liquor only is subjected to the methane fermentation, removing in advance those undigestible solids, a decline in the methane yield is inevitable since only a 50% at maximum out of the total decomposable ingredients could be extracted by treating it with water after simply crushing and mixing them. In addition, it is not easy to dispose of the residue, due to an incomplete extraction of decomposable ingredients.
Previously, the present inventors had found an effective method of treating organic waste that can increase methane yield and can reduce the number of days required for fermentation. The features of this method are to ferment anaerobically organic waste after heating treatment of organic waste acidified by adding acid. (U.S. Application, Ser. No. 651,586: "Method and Apparatus for the Treatment of Waste", filed on Jan. 22, 1976, M. Ishida et al.) now abandoned.
They, also, had found a method that is in no way inferior to the above-mentioned method. The features of this method are to partially decompose the uneasily decomposable materials such as protein, liquid, etc. in organic waste by using alkali instead of the acid. (U.S. Application, Ser. No. 843,262: "Process for Anaerobic Digestion of Biochemical Waste", filed on Oct. 22, 1977, M. Ishida et al.) now abandoned.